First developed in 1956 (see U.S. Pat. No. 2,900,150), rocket catapults comprise generally an outer catapult tube within which is a rocket motor tube containing a solid fuel rocket motor, or propellant grain, which powers the unit in free flight. The two tubes are connected in telescoping relationship, with the catapult tube mounted to the aircraft body and the rocket motor tube fastened to the seat, and during the catapult phase of its operation, the two tubes telescope apart to propel the man-seat mass up the guide rails.
During the catapult stroke, powered by the catapult propellant or cartridge, hot high pressure gases are generated which act against a lower piston face of the rocket motor tube to drive it upward. At a predetermined point in the catapult stroke, the hot catapult gases are vented into the rocket motor tube such that they contact and ignite the rocket motor grain to commence the second phase of the unit's operation. Usually, one or more auxiliary igniters are provided in proximity to the rocket motor grain which are more readily ignited by the hot catapult gases than the grain itself and which aid ignition of the grain. Generally, such auxiliary igniters are simply cavities of granular heat sensitive material, but in certain cases auxiliary igniters have been used which are responsive to the pressure of the incoming gas. In such instances, the pressure actuates a firing pin which fires auxiliary igniter material through a primer train. Both types of auxiliary igniter are therefore dependent upon the state (i.e. the temperature and pressure) of the gas ported into the rocket motor tube to achieve rocket motor ignition.
Two slightly different forms of rocket catapult are in general use today, both of which are designed and operate in accordance with the foregoing description and principles. One such type, illustrated in the above-mentioned U.S. Pat. No. 2,900,150, employs a so-called "tang lock" which both forms the piston face against which the catapult gases act during the first phase of operation and which also serves to lock the inner and outer tubes together prior to operation and to unlock them upon actuation of the unit. The tang lock is attached to the lower end of the rocket motor tube until it is stripped off of that tube upon contact with a shoulder formed in the inner wall of the catapult tube, whereupon a passageway is formed to vent the catapult gases up into the rocket motor tube to ignite the rocket motor.
A second form of rocket catapult, illustrated in U.S. Pat. No. 2,954,947, provides a so-called "booster tube" mounted co-axially in the rocket motor tube within the rocket motor grain, and within which the catapult propellant is provided. The internal volume of the booster tube is initially isolated from the area of the rocket motor grain, and the hot catapult gases are vented down through one port of a multi-port nozzle such that they act against a piston face provided immediately below the nozzle during the catapult stroke. A slide valve in the booster tube, initially closed, is actuable through a lost motion linkage, including a wire cable extending from within the booster tube through the open nozzle port and connected to the base of the catapult tube. As the rocket motor tube accelerates up the catapult tube, it reaches a certain point where the cable is pulled taut and actuates the slide valve to port the hot catapult gases in the booster tube out into the area of the rocket motor grain, causing rocket motor ignition.
Inasmuch as rocket catapults are used almost exclusively in military aircraft, they must be capable of operation under a wide variety of conditions, including a temperature range of from -65.degree. to about 200.degree. F. They are also required to undergo severe vibration testing and, in addition, must not exceed maximum limits of acceleration or rate of change of acceleration (generally referred to as rate of onset); otherwise, the forces applied to the crew member during ejection may severely injure him.
The limitations on maximum acceleration and rate of onset become especially critical at high operating temperatures, when the gas generated by the catapult cartridge or propellant is at maximum temperature and pressure, leading to a fast catapult stroke and rapid ignition of the rocket motor. On the other hand, enough gas pressure must be generated by the catapult or propellant so that even at low operating temperatures, the relatively low pressure and temperature gas relied upon to ignite the rocket motor is capable of doing so prior to separation of the catapult and rocket motor tubes. Because all rocket catapults to date have relied upon the hot catapult gases to ignite the rocket motor, it has at times been difficult to design particular units to obtain proper operation within the severe specification limits applicable. Accordingly, it is an object of the present invention to provide an improved rocket catapult in which rocket motor ignition is achieved by a directly mechanically actuated auxiliary igniter, such that rocket motor ignition is not dependent in any way upon the pressure or temperature of the catapult gases for ignition. This is achieved in accordance with the invention in a manner fully consistent with the severe environmental and operational requirements applicable to rocket catapults.